The present invention relates to a propulsion system for high speed marine craft. In particular, the invention concerns an improved surface drive propulsion system having high operational efficiency.
Known propulsion systems for high speed craft display considerable limitations in performance at low speeds, at high speeds, or throughout the desired speed range. The simplest form of propulsion for marine craft, the submerged propeller, has many limitations and tends to have low efficiency characteristics at high speeds. An improved drive system, the `Z` (or stern) drive, introduced in the 1960's, provides improved efficiency at higher speeds for smaller craft. However, at very high speeds problems are experienced with this type of propeller and often a surface-piercing propeller must be fitted instead.
For high craft speeds surface-piercing propellers fitted either to a specialised surface drive system, or to a Z-drive, give the highest efficiencies. However, conventional surface-piercing propellers are extremely power-absorbing at low speeds. One reason for this is that because these propellers are designed to be run semi-immersed their diameter is large compared to a conventional propeller. Thus until the craft has achieved planing speed the propeller is normally excessively immersed such that the flow and torque requirement are excessively high. A second factor, which is less well understood, is that at low speeds and high power the blades are running at a high lift coefficient, the vapour cavity behind the blade is wide and the distance between the external surface of one blade cavity and the propulsive surface of the succeeding blade is small. Thus the blade is effectively pushing against a vapour bubble with an evident loss of thrust. These two factors in particular cause craft fitted with surface drives to have considerable difficulty getting onto the plane which means they have to be fitted with excessively powerful engines. As a result of the limitations imposed by these drives their usage remains restricted and their cost is high. Also, such propellers are normally mounted well behind the hull which renders them vulnerable to damage when manoevreing or at berth. In most cases, the propeller cannot be raised sufficiently to enable the craft to be beached.
In recent years, jet pump drives have also become increasingly employed for two classes of craft: small performance boats and personal water-craft (jet bikes etc.), and larger luxury yachts and performance work-boats. However, jet drives suffer from a number of distinct disadvantages: in practice the efficiency is usually less than 60% and is often less than 50%. Jet drives are also relatively complex and tend to be expensive; installation is also more onerous than for other drives.